Eric Quémerais

Density and temperatures of the upper Martian atmosphere measured by stellar occultations with Mars Express SPICAM

[1] We present one Martian year of observations of the density and temperature in the upper atmosphere of Mars (between 60 and 130 km) obtained by the Mars Express ultraviolet spectrometer Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM). Six hundred sixteen profiles were retrieved using stellar occultations technique at various latitude and longitude. The atmospheric densities exhibit large seasonal fluctuations due to variations in the dust content of the lower atmosphere which controls the temperature and, thus, the atmospheric scale height, below 50 km. In particular, the year observed by SPICAM was affected by an unexpected dust loading around Ls = 130° which induced a sudden increase of density above 60 km. The diurnal cycle could not be analyzed in detail because most data were obtained at nighttime, except for a few occultations observed around noon during northern winter. There, the averaged midday profile is found to slightly differ from the corresponding midnight profile, with the observed differences being consistent with propagating thermal tides and variations in local solar heating. About 6% of the observed mesopause temperatures exhibits temperature below the CO 2 frost point, especially during northern summer in the tropics. Comparison with atmospheric general circulation model predictions shows that the existing models overestimate the temperature around the mesopause (above 80 to 100 km) by up to 30 K, probably because of an underestimation of the atomic oxygen concentration which controls the CO 2 infrared cooling.

Discovery of an aurora on Mars

In the high-latitude regions of Earth, aurorae are the often-spectacular visual manifestation of the interaction between electrically charged particles (electrons, protons or ions) with the neutral upper atmosphere, as they precipitate along magnetic field lines. More generally, auroral emissions in planetary atmospheres “are those that result from the impact of particles other than photoelectrons” (ref. 1). Auroral activity has been found on all four giant planets possessing a magnetic field (Jupiter, Saturn, Uranus and Neptune), as well as on Venus, which has no magnetic field. On the nightside of Venus, atomic O emissions at 130.4 nm and 135.6 nm appear in bright patches of varying sizes and intensities, which are believed to be produced by electrons with energy <300 eV (ref. 7). Here we report the discovery of an aurora in the martian atmosphere, using the ultraviolet spectrometer SPICAM on board Mars Express. It corresponds to a distinct type of aurora not seen before in the Solar System: it is unlike aurorae at Earth and the giant planets, which lie at the foot of the intrinsic magnetic field lines near the magnetic poles, and unlike venusian auroras, which are diffuse, sometimes spreading over the entire disk. Instead, the martian aurora is a highly concentrated and localized emission controlled by magnetic field anomalies in the martian crust.

A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO

Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60–100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar–antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90–120 km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the ‘cryosphere’ above 100 km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40 years ago. HDO/H2O is enhanced by a factor of ∼2.5 with respect to the lower atmosphere, and there is a general depletion of H2O around 80–90 km for which we have no explanation.

Charge-transfer induced EUV and soft X-ray emissions in the heliosphere

We study the EUV/soft X-ray emission generated by charge transfer between solar wind heavy ions and interstellar H and He neutral atoms in the inner Heliosphere. We present heliospheric maps and spectra for stationary solar wind, depending on solar cycle phase, solar wind anisotropies and composition, line of sight direction and observer position. A time-dependant simulation of the X-ray intensity variations due to temporary solar wind enhancement is compared to XMM Newton recorded data of the Hubble Deep Field North observation (Snowden et al. 2004). Results show that the heliospheric component can explain a large fraction of the line intensity below 1.3 keV, strongly attenuating the need for soft X-ray emission from the Local Interstellar Bubble.We study the EUV/soft X-ray emission generated by charge transfer between solar wind heavy ions and interstellar neutral atoms and variations of the X-ray intensities and spectra with the line of sight direction, the observer location, the solar cycle phase and the solar wind anisotropies, and a temporary enhancement of the solar wind similar to the event observed by Snowden et al. (2004) during the XMM-Hubble Deep Field North exposure. Methods.Using recent observations of the neutral atoms combined with updated cross-sections and cascading photon spectra we have computed self-consistent distributions of interstellar hydrogen, helium and highly charged solar wind ions for a stationary solar wind and we have constructed monochromatic emission maps and spectra. We have evaluated separately the contribution of the heliosheath and heliotail, and included X-ray emission of the excited solar wind ions produced in sequential collisions to the signal. Results.In most practicable observations, the low and medium latitude X-ray emission is significantly higher at minimum activity than at maximum, especially around December. This occurs due to a strong depletion of neutrals during the high activity phase, which is not compensated by an increase of the solar wind flux. For high latitudes the emission depends on the ion species in a complex way. Intensity maps are in general significantly different for observations separated by six-month intervals. Secondary ions are found to make a negligible contribution to the X-ray line of sight intensities, because their density becomes significant only at large distances. The contribution of the heliosheath-heliotail is always smaller than 5%. We can reproduce both the intensity range and the temporal variation of the XMM-HDFN emission lines in the 0.52-0.75 keV interval, using a simple enhanced solar wind spiral stream. This suggests a dominant heliospheric origin for these lines, before, during and also after the event.

Vertical distribution of ozone on Mars as measured by SPICAM/Mars Express using stellar occultations

[i] The ultraviolet spectrometer of the SPICAM instrument on board the European Mars Express mission has performed stellar occultations to probe the atmosphere. Vertical profiles of ozone are retrieved from inversion of transmission spectra in the altitude range 20-30 to 70 km. They are analyzed here as functions of latitude and season of the observations. These occultations have been monitored on the night side, from northern spring equinox (L s = 8°) to northern winter solstice (L s = 270°). The profiles show the presence of two ozone layers: (1) one located near the surface, the top of which is visible below 30 km altitude, and (2) one layer located in the altitude range 30 to 60 km, a feature that is highly variable with latitude and season. This layer is first seen after L s = 11°, and the ozone abundance at the peak tends to increase until L s ∼ 40°, when it stabilizes around 6-8 x 10 9 cm -3 . After southern winter solstice (L s ∼ 100°), the peak abundance starts decreasing again, and this ozone layer is no longer detected after L s ∼ 130°. A recent model (Lefevre et al., 2004) predicted the presence of these ozone layers, the altitude one being only present at night. Though the agreement between model and observations is quite good, this nocturnal altitude layer is present in SPICAM data over a less extended period than predicted. Though a possible role of heterogeneous chemistry is not excluded, this difference is probably linked to the seasonal evolution of the vertical distribution of water vapor.

The study of the martian atmosphere from top to bottom with SPICAM light on mars express

Abstract SPICAM Light is a small UV-IR instrument selected for Mars Express to recover most of the science that was lost with the demise of Mars 96, where the SPICAM set of sensors was dedicated to the study of the atmosphere of Mars (Spectroscopy for the investigation of the characteristics of the atmosphere of mars). The new configuration of SPICAM Light includes optical sensors and an electronics block. A UV spectrometer (118–320 nm, resolution 0.8 nm) is dedicated to Nadir viewing, limb viewing and vertical profiling by stellar occultation (3.8 kg). It addresses key issues about ozone, its coupling with H2O, aerosols, atmospheric vertical temperature structure and ionospheric studies. An IR spectrometer (1.2– 4.8 μm , resolution 0.4–1 nm) is dedicated to vertical profiling during solar occultation of H2O, CO2, CO, aerosols and exploration of carbon compounds (3.5 kg). A nadir looking sensor for H2O abundances (1.0– 1.7 μm , resolution 0.8 nm) is recently included in the package (0.8 kg). A simple data processing unit (DPU, 0.9 kg) provides the interface of these sensors with the spacecraft. In nadir orientation, SPICAM UV is essentially an ozone detector, measuring the strongest O3 absorption band at 250 nm in the spectrum of the solar light scattered back from the ground. In the stellar occultation mode the UV Sensor will measure the vertical profiles of CO2, temperature, O3, clouds and aerosols. The density/temperature profiles obtained with SPICAM Light will constrain and aid in the development of the meteorological and dynamical atmospheric models, from the surface to 160 km in the atmosphere. This is essential for future missions that will rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow study of the ionosphere through the emissions of CO, CO+, and CO2+, and its direct interaction with the solar wind. Also, it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere. The SPICAM Light IR sensor is inherited from the IR solar part of the SPICAM solar occultation instrument of Mars 96. Its main scientific objective is the global mapping of the vertical structure of H2O, CO2, CO, HDO, aerosols, atmospheric density, and temperature by the solar occultation. The wide spectral range of the IR spectrometer and its high spectral resolution allow an exploratory investigation addressing fundamental question of the possible presence of carbon compounds in the Martian atmosphere. Because of severe mass constraints this channel is still optional. An additional nadir near IR channel that employs a pioneering technology acousto-optical tuneable filter (AOTF) is dedicated to the measurement of water vapour column abundance in the IR simultaneously with ozone measured in the UV. It will be done at much lower telemetry budget compared to the other instrument of the mission, planetary fourier spectrometer (PFS).

Interplanetary Lyman α line profiles derived from SWAN/SOHO hydrogen cell measurements: Full‐sky Velocity Field

We present here an analysis of 1 year of data obtained by the solar wind anisotropies (SWAN) instrument on board the SOHO spacecraft orbiting around the Ll Lagrange point at 1.5 × 106 km sunward from Earth. This instrument is measuring the interplanetary Lyman α background due to solar photons backscattered by hydrogen atoms in the interplanetary medium. The interplanetary (IP) Lyman a line profile reflects the velocity distribution of H atoms projected onto the line of sight (LOS). Here we apply a new profile reconstruction technique using data from the two hydrogen absorption cells included in the SWAN instrument. For a LOS in a fixed celestial direction, the Doppler shift between the interplanetary emission profile and the H cell absorption profile varies by up to ±0.12 A during 1 year, owing to the Earths orbital velocity around the Sun, equal to 30 km s−1. Such a Doppler spectral scan across the emission line allows us to derive Lyman α line profiles, and hence the velocity distribution, in and out of the ecliptic independent of any modeling of the neutral hydrogen atom distribution in the heliosphere or of the multiple scattering of solar photons. The spatial distribution of the apparent velocity relative to the Sun as observed from the orbit of SOHO is derived for all directions, except within 5° of the ecliptic poles. This determination strongly constrains models of the interaction of the interstellar hydrogen with the solar wind. New estimates of the upwind direction (252.3° ± 0.73° and 8.7° ± 0.90° in J2000 ecliptic coordinates) show a small discrepancy by 3° – 4° with the direction of the helium flow, perhaps connected with an asymmetry of the heliosphere induced by the interstellar magnetic field. We find that the apparent velocity relative to the sun in the upwind direction is −25.4 ± 1 km/s, whereas it is equal to 21.6 ± 1.3 km s−1 in the downwind direction. A preliminary analysis shows that the Zero Doppler shift cone and the difference between the upwind and downwind velocities correspond to a ratio μ of Lyman α radiation pressure to solar gravity of 0.9–1.0. It follows that the observed upwind apparent velocity is compatible with a velocity at infinity of H atoms of the order of 21–22 km s−1. However, extensive modeling is required in order to get more definite conclusions. The velocity map presented here is the first ever obtained. For this reason, we discuss in detail the Doppler spectral scan method and the H cell data.

LYMAN-ALPHA OBSERVATIONS OF COMET HYAKUTAKE WITH SWAN ON SOHO

Abstract The SWAN instrument on board SOHO is a Lyman-α (Lα) photometer able to map the sky intensity with a resolution of 1°, and a capability of microstepping (0.1°). SWAN is primarily devoted to the study of the large scale distribution of solar wind from its imprints on the interplanetary sky background, but was in addition extensively used to map the Lα emission of several comets since its launch in December 1995. Here we report observations of comet C/1996 B2 (Hyakutake). Its Lα emission cloud extended over more than 60° while approaching the Earth at 0.102 AU. A comparison with a simple model allowed hydrogen and H2O production rates to be derived, while the comet approached closer to the Sun from 1.12 AU to 0.53 AU distance to the Sun, pre-perihelion. The derived H2O production rate was found in fair agreement with other derivations (IUE and ground-based in the IR and UV), validating the Lα method. The H2O production by SWAN was related to several other measurements of minor constituents in order to derive new values of abundance of CO, HCN, H2CO, CH3OH and CH3CN. Most important, the D H ratio in comet Hyakutake is now found at 3 × 10−4, as in comet Halley, while a previous estimate based on a wrong H2O number had indicated a value twice lower, with important cosmogonic consequences. The time evolution showed a fast surge on 21 March, coinciding with the time of fragmentation of the nucleus as detected 3 days later at Pic du Midi. This surge is also confirmed by the detailed comparison of H column densities (observed vs model) as a function of the distance to the nucleus, showing a larger ratio in the inner region (younger atoms) than in the outer region (older atoms) on 21 March, and then a progressive filling-in of the H envelope. After the surge, there was a plateau for 16 days around 1.8 × 1029 H2O mol s−1, and then an increase following approximately a R−2 law. This behavior is interpreted as the surge and plateau corresponding to the fragmentation and total disruption/evaporation of a fragment of the nucleus, of approximately 200 m. Finally, it is argued that the first detection of ethane C2H6 in this comet (IR observations) might have been the result of the special circumstances (a large fragment disrupted very near the Earth) rather than revealing a new special class of ethane-rich comets as argued by other authors.

HST/GHRS Observations of the Velocity Structure of Interplanetary Hydrogen

We present high-resolution spectra of the emission-line profile of inflowing interplanetary hydrogen atoms along lines of sight with the Earth orbital motion upwind (into the flow), downwind, and across the flow to Doppler-shift the line from the geocoronal emission. The line-center positions, in comparison with hot-model profiles, confirm that the inflow speed of H atoms far from the Sun (~50 AU) is in the range 18-21 km s-1, which implies a decrease in the velocity distribution of 5-8 km s-1 for hydrogen within the solar system, relative to the He flow and to the local interstellar medium. Best-fit values are derived for the speed and effective solar gravity along the three lines of sight by comparison with model profiles convolved with the instrument line-spread function. For the assumed inflow direction, the cross-flow line profile requires that the μ-value be slightly less than unity near solar minimum, and a technique is presented for determining the exact inflow direction and μ-value independently of the other parameters. The line widths indicate a broadening along the flow direction in addition to the dynamical effects near the Sun expected from two different hot models, whereas the cross-flow line width is similar to the hot-model profiles. The altered velocity distribution in the inflow direction appears likely to be related to the crossing of the interstellar/interplanetary medium interface structure, although questions remain about the cumulative effects of changing solar activity on the timescale of the H atom flow through the solar system.

A rapid decrease of the hydrogen corona of Mars

Mars is believed to have lost much of its surface water 3.5 billion years ago, but the amounts that escaped into space and remain frozen in the crust today are not well known. Hydrogen atoms in the extended martian atmosphere, some of which escape the planets gravity, can be imaged through scattered solar UV radiation. Hubble Space Telescope (HST) images of the ultraviolet H Ly α emission now indicate that the coronal H density steadily decreased by a factor of roughly 40% over 4 weeks, a far greater variation than had been expected. The leading candidate cause is a decrease in the source rate of water molecules from the lower atmosphere, consistent with seasonal changes and a recent global dust storm. This implies that the rate of escape of martian hydrogen (and thereby water) into space is strongly dependent on the lower atmospheric water content and distribution.